Non-oriented electrical steel sheet excellent in core loss and manufacturing method thereof

ABSTRACT

A non-oriented electrical steel sheet is characterized in that the number density of inclusions with an equivalent volume diameter of less than 100 nm contained in the steel sheet is 1×10 10  [/mm 3 ] or less, and that the steel sheet contains, by mass %, C: up to 0.01%, Si: 0.1% to 7.0%. Al: 0.1% to 3.0%. Mn: 0.1% to 2.0%, REM: 0.0003% to 0.05%. Ti: up to 0.02%. S: up to 0.005%. and N: up to 0.005%. the balance Fe and inevitable impurities and the mass % of Al represented by [Al] and the mass % of Ti represented by [Ti] satisfy the equation log([Ti]×[N])−1.19×log([Al]×[N])+1.84&gt;0 . . . (1).

FIELD OF THE INVENTION

The present invention relates to a non-oriented electrical steel sheetused for a magnetic core of a motor or the like and a manufacturingmethod thereof, and more specifically, to a non-oriented electricalsteel sheet excellent in core loss, especially in core loss after stressrelief annealing.

BACKGROUND INFORMATION

A conventional non-oriented electrical steel sheet generally has aminimum core loss at a grain size of about 150 μm. Thus, a steel sheetof a better grain growth during finish annealing can be desired in viewof the product property, simplified manufacturing process, and highproductivity.

For example, a preferable grain size may be less than, for example, 40μm when the steel sheet is subjected to punching for magnetic cores bycustomers because the accuracy in punching is better when the grain inthe sheet is finer. As described above, preferences for core loss andthe punching accuracy to the grain size may conflict with one another.

When these conflicting requirements are satisfied, a product sheet canbe shipped with a small grain size and may be subjected to stress reliefannealing, for example, at 750° C. for about two hours to grow thegrains after punching by the user or consumers. Recently, the customershave had strong demands for materials with low core loss. In addition,there may be an increasing need for the product sheets which have abetter grain growth during stress relief annealing since a reduction inthe stress relief annealing time has been demanded because ofimprovement in productivity by the consumers.

One of major factors of inhibiting grain growth is the dispersion offine inclusions in the steel. It is known that the grain growth is moreinhibited with larger number and smaller size of the inclusions in theproduct.

In particular, as presented by Zener, the grain growth deterioratesfurther with a smaller r/f value that is represented by the equivalentvolume radius r and the volume fraction f of inclusions in the steel.Accordingly, it is important not only to decrease the number, but alsoto increase the size of the inclusions for a good grain growth of thesteel.

For preferable ranges of the size and number of inclusions in thenon-oriented electrical steel, for example, Japanese Patent ApplicationLaid-open No. 2001-271147, the entire disclosure of which isincorporated herein by reference, describes that inclusions with a sizefrom 0.1 [μm] to 1 [μm] and inclusions with a size of greater than 1[μm] are contained within a range from 5000 [/mm²] to 10⁵ [/mm²] and arange of 500 [/mm²] or less, respectively, per unit cross-section area.

For example, the number of inclusion per unit cross-section area can beconverted to the number per unit volume. The above-indicated ranges canbe converted to the range of 5×10⁶ [/mm³] to 1×10⁹ [/mm³] and the rangeof 5×10⁵ [/mm³] or less, respectively

Inclusions inhibiting grain growth in a non-oriented electrical steelsheet are, e.g., oxides such as silica and alumina, sulfide such asmanganese sulfide, and nitrides such as aluminum nitride and titaniumnitride.

Highly purified molten steel generally provides a steel sheet free fromthese inclusions. There are several methods to reduce detrimentaleffects of the inclusions by adding various elements to the moltensteel.

For oxides, a technological progress allows a removal of oxides frommolten steel by adding a sufficient amount of Al, a strong deoxidizer,and stirring enough periods to float them up for removal.

For sulfides, in order to remove sulfur from molten steel thoroughly,the methods of adding of some rare earth metals as desulfurizer to fixsulfur in the steel is described in, for example, Japanese PatentApplication Laid-open No. Sho 51-62115, Japanese Patent ApplicationLaid-open No. Sho 56-102550, and Japanese Patent Publication No.3037878, the entire disclosures of which are incorporated herein byreference. Further, for nitrides, the methods of adding boron that leadto the formation of coarse BN inclusion in the steel and the preventionof finer other inclusions are described in Japanese Patent PublicationNo. 1167896 and Japanese Patent Publication No. 1245901, the entiredisclosures of which are incorporated herein by reference.

However, the high purification in the stage of molten steel may not bepreferable because of unavoidable increased steelmaking cost. On theother hand, the above-described methods of adding elements areinsufficient in improvement of grain growth and core loss in finishannealing or stress relief annealing after punching at loweredtemperature and reduced period.

Even when the number density of inclusions is adjusted to fall withinthe recommended range described in Japanese Patent Application Laid-openNo. 2001-271147, the grain growth would likely still be unimproved insome cases where the stress relief anneal is performed at a lowertemperature and for a shorter period.

This may be because the size and the number density of inclusionsadjusted based on the conventional knowledge are different from thecomposition, the size, and the number density of inclusions actuallyinhibiting grain growth as described herein below.

SUMMARY OF THE INVENTION

One of the objects of the present invention is to provide a non-orientedelectrical steel sheet which provides large grain size enough to lowercore loss and, e.g., to lower core loss even by annealing at a lowertemperature and for a shorter period after punching.

According to one exemplary embodiment of the present invention, anon-oriented electrical steel sheet excellent in core loss is provided.For example, a number density of inclusions with an equivalent volumediameter of less than 100 nm contained in the steel sheet is 1×10¹⁰[/mm³] or less.

According to another exemplary embodiment of the present invention,another non-oriented electrical steel sheet excellent in core loss isprovided. For example, a number density of inclusions with an equivalentvolume diameter of less than 50 nm contained in the steel sheet is2.5×10⁹ [/mm³] or less.

The steel sheet may contain, by mass %, C: up to 0.01%, Si: 0.1% to7.0%, Al: 0.1% to 3.0%, Mn: 0.1% to 2.0%, REM: 0.0003% to 0.05%, Ti: upto 0.02% or less, S: up to 0.005%, and N: up to 0.005%, the balance Feand inevitable impurities, and that the mass % of Al, the mass % of Nand the mass % of Ti can satisfy the following equation:log([Ti]×[N])−1.19×log([Al]×[N])+1.84>0

The steel sheet may further contain, by mass %, one or more of P: up to0.1%, Cu: up to 0.5%, Ca or Mg: up to 0.05%, Cr: up to 20%, Ni: up to1.0%, a total of one or two of Sn and Sb: up to 0.3%, Zr: up to 0.01%,V: up to 0.01%, O: up to 0.005%, and B: up to 0.005%.

According to yet another exemplary embodiment of the present invention,a method for manufacturing a non-oriented electrical steel sheetexcellent in core loss can be provided. In particular, a steel may bemaintained within a temperature range of 1200° C. to 1300° C. for oneminute or more. Fir example, the steel may contain, by mass %, C: up to0.01%, Si: 0.1% to 7.0%, Al: 0.1% to 3.0%, Mn: 0.1% to 2.0%, REM:0.0003% to 0.05%, Ti: up to 0.02%, S: up to 0.005%, and N: up to 0.005%,the balance Fe and inevitable impurities. The mass % of Al representedby [Al], the mass % of N and the mass % of Ti represented by [Ti] can beprovided as follows:log([Ti]×[N])−1.19×log([Al]×[N])+1.84>0

The steel may further contain, by mass %, one or more of P: up to 0.1%,Cu: up to 0.5%, Ca or Mg: up to 0.05%, Cr: up to 20%, Ni: up to 1.0%, atotal of one or two of Sn and Sb: up to 0.3%, Zr: up to 0.01%, V: up to0.01%, O: up to 0.005%, and B: up to 0.005%.

These and other objects, features and advantages of the presentinvention will become apparent upon reading the following detaileddescription of embodiments of the invention, when taken in conjunctionwith the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the invention will becomeapparent from the following detailed description taken in conjunctionwith the accompanying figure showing illustrative embodiments, resultsand/or features of the exemplary embodiment(s) of the present invention,in which:

FIG. 1 is a graph showing an exemplary relation between the numberdensity of inclusions with a size of less than 100 nm and the grain sizeand core loss value after annealing according to one exemplaryembodiment of the present invention;

FIG. 2 is a graph showing an exemplary relation between the numberdensity of inclusions with a size of less than 50 nm and the grain sizeand core loss value after annealing according to another exemplaryembodiment of the present invention;

FIG. 3 is an exemplary illustration of composite TiN inclusions aroundREM oxysulfide according to an exemplary embodiment of the presentinvention;

FIG. 4 is a graph showing an exemplary relation between the indexobtained by a predetermined technique and presence or absence of fineTiN according to still another exemplary embodiment of the presentinvention; and

FIG. 5 is a graph showing an exemplary relation between the indexobtained by the predetermined technique and the grain size and core lossvalue after annealing according to yet another exemplary embodiment ofthe present invention.

DETAILED DESCRIPTION

The magnetic property of a non-oriented electrical steel sheet isgenerally affected by fine inclusions contained in the steel sheet andnewly found appropriate ranges of the size and the number density ofinclusions to provide an excellent magnetic property and a punchingproperty.

The effects exerted on the magnetic property by the size and the numberdensity of inclusions will be described using the steels shown below.The steel portions shown in the drawings of the present application aremerely exemplary in nature, and in no way limit the present invention.

For example, certain steel portions that contain C, Si, Al, Mn, REM, Ti,S, N and the balance Fe with inevitable impurities were continuouslycast and hot-rolled, and the hot rolled sheets were subjected to hotband annealing, cold rolling into a thickness of 0.5 mm and finishannealing at 850° C. for 30 seconds. The steel sheet can be furthercoated with an insulative coating, and the product sheets may besubjected to stress relief annealing at 750° C. for approximately 1.5hours.

Further, the number density of inclusions, grain size and core loss ofthe product sheets after the stress relief anneal can be evaluated. Theexemplary results are shown in Table 1 and FIGS. 1 and 2.

As shown in Table 1 and FIG. 1, there is a correlation between the grainsize and core loss after the anneal and the number density (e.g., thenumber per 1 mm3) of inclusions with an equivalent volume diameter(e.g., the equivalent volume diameter of inclusions relating to thepresent invention will be referred to as “inclusion size” or just“size”) of less than 100 nm, and the grain growth and core loss islikely excellent when the number density of inclusions is 1×10¹⁰ [/mm³]or less.

Further, as shown in FIG. 2, when the number density of inclusions witha size of less than 50 nm is 2.5×10⁹ [/mm³] or less, the magneticproperties are likely remarkably excellent.

On the other hand, when the number density of inclusions with a size of100 nm or greater is 1×10⁹ [/mm³] or less but the number density ofinclusions with a size of less than 100 nm is greater than 1×10¹⁰[/mm³], the magnetic properties may be less desirable.

For example, many inclusions less than 100 nm may be detected also inthe samples whose inclusions with a size of 0.1 μm (=100 nm) or greaterare 1×10⁹ [/mm³] or less. Thus, it can be determined that fineinclusions with a size of less than 100 nm, particularly a size of lessthan 50 nm are, e.g., a major factor of grain growth inhibition whichleads to deteriorate core loss.

The above results are for the case where stress relief annealing may beperformed at 750° C. for 1.5 hours, a period shorter than typical stressrelief annealing condition, at 750° C. for 2 hours. If the stress reliefanneal is performed in a typical condition, the difference of graingrowths and core losses among samples described above can be clearerbecause the difference of the pinning effect of fine inclusions becomeslarger in such condition.

Thus, the desirable product properties may not be readily obtained byonly specifying the number density of inclusions with a size of 100 nmor greater referred to conventionally. Further, the preferable magneticproperties of the product in accordance with one exemplary embodiment ofthe present invention can be obtained by specifying the number densityof inclusions with a size of less than 100 nm, and that more preferableproperties can be obtained by specifying the number density ofinclusions with a size of less than 50 nm.

The exemplary excellent properties of the steel portions in accordancewith an exemplary embodiment of the present invention can be exhibitedif the size and the number density of inclusions in a steel sheet aresatisfied, e.g., regardless of compositions of the steel particularly.

The exemplary embodiment of the method according to the present can beperformed for the above-described evaluation. For example, a productsheet used for a sample may be polished to an appropriate thickness fromits surface to make a mirror surface and subjected to etching describedbelow, and thereafter its replica can be taken so that inclusionstransferred to the replica may be observed under a field emission-typetransmission electron microscope. In such case, instead of a replica, athin film may be prepared for observation.

The size and the number density of inclusions can be measured for manyor all of the inclusions within a certain observation area. Thecomposition of the inclusions may be determined by an energy dispersiveX-ray analyzer and diffraction pattern analysis.

For the smallest size of the inclusions, e.g., there may be no inclusionsmaller than the lattice constant of the inclusion which is aboutseveral angstroms. The lowest limit of the size of the stably existinginclusion nuclear can be about 5 nm. Therefore, any method (for example,magnification) capable of observing the inclusion having a size at thatlevel can be suitable for the observation.

An exemplary etching method can be employed which was proposed byKurosawa et al (Fumio Kurosawa, Isao Taguchi, and Ryutaro Matsumoto:Journal of The Japan Institute of Metals, 43 (1979), p. 1068), theentire disclosure of which is incorporated herein by reference. Usingsuch exemplary method, the samples may be subjected to electrolyteetching in a non-aqueous solvent to dissolve only the steels withinclusions being left, thereby the inclusions can be filtrated.

The evaluation results of the compositions of fine inclusions in theabove-described exemplary product steels containing Ti by theaforementioned method can indicate that the major inclusions (50% ormore in the number of pieces) with a size of less than 100 nm are Ticompounds such as TiN, TiS, or TiC.

In an electrical steel, the starting temperatures of formation of TiN,TiS, and TiC can fall within 1200 to 1300° C., 1000 to 1100° C., and 700to 800° C., respectively. For example, TiN precipitates during coolingprocess after casting of a slab or the like, while TiS and TiCprecipitate during the process but melt again at the heating temperaturein a normal hot-rolling step and re-precipitates during coolingthereafter.

In the latter case, since the diffusivity of Ti in the steel at aroundthe starting temperature of the Ti precipitations is lower than that ofother metal elements, the Ti compounds may not sufficiently growcompared to other inclusions. As a result, the size of the Ti compoundmay not be 100 nm or greater, but is less than 100 nm, or less than 50nm in some cases, resulting in fine Ti compound.

The number density of inclusions increases as the inclusion size becomessmaller and accordingly the grain growth can be inhibited more strongly.However, the major inclusions which strongly inhibit grain growth in theelectrical steel can be fine inclusions with a size of less than 100 nm,therefore limiting the number density of the inclusions remarkablyimproves the grain growth and consequently core loss, and that the mostof the inclusions with a size of less than 100 nm are Ti compounds suchas, for example, TiN, TiS, or TiC.

It is generally difficult to prevent a contamination of a small amountof Ti in the steel manufacturing process. Since steels containing aconsiderable amount of Ti are also produced besides the electrical steelin a normal steelmaking process, Ti may inevitably contaminate into thesteel from steel, slag or like adhering to a refractory.

Even in the manufacturing process of only electrical steel Ti may comeinto the steel from, for example, a ferrosilicon alloy used foradjusting Si composition in the steel, and/or from a reduced slagcontaining Ti oxide.

Although it has been commonly known that a small amount of Ti inevitablycontaminated in steel inhibits grain growth, a non-oriented electricalsteel sheet with better grain growth can be obtained by controlling theamount of Ti in the steel within a preferable range, by allowinginevitably contaminated Ti or even by intentional addition of Ti,according to an exemplary embodiment of the present invention.

The effects of steel composition on the inclusions are described below.For example, to render the detrimental effects of TiS among Ticompounds, a technique using REM (as described herein below), decreasingsulfide-based inclusions by fixing S by addition of REM has been used.

Hereinafter, the term of “REM” refers to, e.g., 17 elements, in total,including 15 elements of lanthanum with an atomic number of 57 tolutetium with an atomic number of 71, and scandium with an atomic numberof 21 and yttrium with an atomic number of 39.

It has been determined according to an exemplary embodiment of thepresent invention that Ti in the steel can be free from detrimentaleffects if Ti is within an appropriate composition range where REMoxysulfide or REM sulfide allow restraint of fine sulfides by fixationof sulfur in the steel as TiS. Furthermore fine TiN can be restrained bycompositely precipitating and growing of TiN on a surface of REMoxysulfides or REM sulfides, as described below.

The starting temperatures of precipitation of TiN and AIN are close toeach other in the electrical steel. Since Al is overwhelmingly superiorin amount, if the starting temperature of precipitation of AIN is higherthan that of TiN even slightly, N in the steel preferentially bonds withAl and is thereby consumed for precipitation of AIN, while Ti, which isless in amount than Al, has remarkably less opportunity to bond with N.

Because of shortage of the amount of N, fine dispersed TiN in the steelcan be caused by a deprived chance of nucleation and growing of TiN on asurface of REM oxysulfide or REM sulfide.

Accordingly, the requirement affecting the precipitation of fine TiN isthe precipitation temperature of TiN exceeding that of AIN which can bedetermined by a solubility product.

For example, the temperatures of production of TiN and AIN correlate to[Ti]×[N] and [Al]×[N], respectively, where [Ti] represents the mass % ofTi, [N] represents the mass % of N, and [Al] represents the mass % ofAl.

According to an exemplary embodiment of the present invention, it hasbeen determined that when the REM amount falls within a range of 0.0003to 0.05 mass % and the components satisfy the following equation (1), Tiis scavenged as TiN by REM oxysulfide or REM sulfide, resulting inrestraint in precipitation of finely dispersed TiN.log([Ti]×[N])−1.19×log([Al]×[N])+1.84>0  (1)

Consequently, the number density of inclusions with an equivalent volumediameter of less than 100 nm contained in the steel sheet can be 1×10¹⁰[/mm³] or less, or the number density of inclusions with an equivalentvolume diameter of less than 50 nm contained in the steel sheet can be2.5×10⁹ [/mm³] or less. Therefore, a non-oriented electrical steel sheetcan be provided in which the grain growth is better in typical usedannealing condition and thereby the annealing time can be shortened. Inparticular, an excellent core loss can be obtained in the stress reliefanneal at a lower temperature for a shorter period. Further low coreloss can be attained by annealing at conventional general stress reliefannealing conditions, 750° C. for 2 hours.

The effects and other features of the present invention will bedescribed in detail below with reference to, e.g., Table 2.

For example, in Table 2, No. 11 is referred to a steel containing, bymass %, C: 0.0024%, Si: 2.1%, Al: 0.32%, Mn: 0.2%, S: 0.0025%, Ti:0.0016%, N: 0.0019%, and REM: 0.0045%.

No. 12 to No .20 were steels containing, by mass %, C: 0.0024%, Si:2.1%, Mn: 0.2%, S: 0.0025%, P: 0.02%, Cu: 0.01%, and various amounts ofAl, Ti, N, and REM as shown in Table 2.

These exemplary steels were subjected to continuous casting, hotrolling, hot-rolled sheet annealing, cold rolling to a thickness of 0.50mm, finish annealing at 850° C. for 30 seconds, and coated with aninsulative coating. The grain size of any of the product sheets waswithin a range of 30 to 33 μm.

Such product sheets were then subjected to stress relief annealing at750° C. for 1.5 hours, a shorter time than usual. Thereafter, the grainsizes and the magnetic properties were evaluated. The results are shownin, e.g., Table 2 and FIGS. 3 and 4.

As provided in Nos. 11 to 14 of Table 2, when the compositions of thesamples were appropriate and the amounts of inclusions were within therange according to the present invention, the grain sizes after thestress relief annealing ranged from 67 to 71 μm and the magneticproperties (core loss: W15/50) were excellent such as 2.7 [W/kg] or lessbecause of the good grain growth.

As a result of evaluation of the size by the above-described exemplarymethod according to the present invention, the number density, and thecomposition of inclusions in the product sheets, it was shown that0.6×10¹⁰ [/mm³] of MnS with a size of less than 100 nm existed in No.11, and 0.3×10¹⁰ to 0.5×10¹⁰ [/mm³] of Cu₂S with a size of less than 100nm existed in Nos. 12 to 14, so the number densities of inclusions were1.0×10¹⁰ [/mm³] or less in the samples. Further, REM oxysulfide or REMsulfide with a size of 0.2 [μm] to 2.0 [μm] existed in the productsheets.

An example of REM oxysulfide is shown in FIG. 3. As shown in FIG. 3,e.g., TiN compositely precipitated around the inclusions containing REMand became large.

As described above, REM in the steel forms REM oxysulfide or REM sulfidefixing S resulting in hindering or suppressing fine sulfide formation,and that TiN with a size of greater than several tens nm compositelyprecipitated on REM oxysulfide or REM sulfide scavenges Ti, wherebyformation of fine inclusions containing Ti is prevented.

As provided in No. 15 of Table 2, although the REM amount was within arange of 0.0003 to 0.05 mass %, the Ti amount exceeded 0.02 mass %.Thus, 2.5×10¹⁰ [/mm³] of TiS with a size of less than 100 nm existed inthis exemplary product sheet, thereby inhibiting grain growth and, as aresult, the grain size after stress relief annealing remainedapproximately 35 [μm] and the W15/50 value was approximately 3.06[W/kg].

In this case, TiN adhered to REM oxysulfide or REM sulfide was observedas the inclusion with a size of greater than 100 nm. Therefore, thescavenge effect of Ti was exerted as described above. However the Ticould not completely be scavenged by REM oxysulfide or REM sulfidebecause there was excessive Ti, resulting in Ti remaining in the steel.It can be considered that a considerable amount of TiC precipitated fromsuch un-scavenged Ti in the steel in the temperature history after thehot-rolling step. Consequently, the upper limit of the Ti amount canpreferably be 0.02 mass %.

In each of Nos. 16 to 18 of Table 2, the REM amount was within a rangeof 0.0003 to 0.05 mass %, and the Ti amount was 0.02 mass % or less, butthe composition were out of the ranges defined by the evaluationequation (1), and therefore AIN was observed as inclusions with a sizeof greater than 100 nm in each of the exemplary product sheets.

Further, 1.6 to 1.8×10¹⁰ [/mm³] of TiN with a size of less than 100 nmwas in exist Accordingly, the grain sizes after stress relief annealingremained between approximately 38 [μm] and 41 [μm], and the W15/50 werebetween approximately 2.76 [W/kg] and 2.83 [W/kg].

Further, the relation between the value on the left side of the equation(1) and the presence or absence of fine TiN less than 100 nm is shown inFIG. 4. As provided in FIG. 4, the formation of fine TiN can besuppressed when the equation (1) is satisfied.

Further, the relation between the value on the left side of the equation(1) and the grain size and the core loss after annealing is shown in theexemplary graph of FIG. 5. As provided in FIG. 5, the grain growth canbe good, and the core loss may be excellent when the equation (1) issatisfied.

For example, when the Ti amount is small as shown in Nos. 17 and 18 ofTable 2, fine TiN precipitates by contrast. This can be because when Tiis deficient, AIN preferably precipitates as is also indicated by theequation (1).

It is know that the Ti amount should be preferably as small as possible,and therefore contamination of Ti into the steel can be intensivelyprevented. However, according to an exemplary embodiment of the presentinvention, such large amount of labor for decreasing Ti is notnecessary. For example, it is preferable to make the Ti amount in thesteel possibly larger than the Ti amount inevitably contaminatedtherein, by adding Ti. This can cause TiN to compositely precipitate onREM oxysulfide or REM sulfide and to be scavenged from the steel, sothat TiN does not re-melt and finely reprecipitate alone in the thermalhistory after hot rolling. This can result in an increased flexibilityto set the hot-rolling schedule as well as possibility to obtain anexcellent product property. In particular, to obtain an electrical steelwith good grain growth and excellent core loss property, according to anexemplary embodiment of the present invention, the Ti amount can be lessrestricted or controlled to fall within the above-described preferablerange.

Further, on the condition that the starting temperature of precipitationof TiN surely exceeds the starting temperature of precipitation of AIN,the formation of fine TiN can be stably suppressed. The difference instarting temperature of formation of TiN and AIN may be about 10° C. orhigher.

The condition to achieve the difference is that the contents of Ti, N,and Al can satisfy the following equation (2).log([Ti]×[N])−1.19×log([Al]×[N])+1.70>0  (2)

In particular, [Ti] represents the mass % of Ti, [N] represents the mass% of N, and [Al] represent the mass % of Al.

The difference of more than about 15° C. in the precipitation startingtemperature between TiN and AlN can be more preferable because theprecipitation starting temperature of TiN more surely exceeds that ofAIN so that the formation of fine TiN can be suppressed more stably.

The condition to achieve the difference can be obtained with thecontents of Ti, N, and Al satisfying the following equation (3).log([Ti]×[N])−1.19×log([Al]×[N])+1.58>0  (3)

Again, [Ti] represents the mass % of Ti, [N] represents the mass % of N,and [Al] represent the mass % of Al.

Further, the difference of more than about 20° C. in the precipitationstarting temperature between TiN and AIN can be further preferablebecause the precipitation starting temperature of TiN further surelyexceeds that of AIN so that the formation of fine TiN can be suppressedfurther stably.

The condition to achieve the difference can be obtained with thecontents of Ti, N, and Al satisfying the following equation (4).log([Ti]×[N])−1.19×log([Al]×[N])+1.49>0  (4)

The present inventors have also found that the contents of Ti, N, and Almore preferably satisfy the following equation (5).log([Ti]×[N])−1.19×log([Al]×[N])+1.35>0  (5)

For example, no REM was added in No. 19 of Table 2, while the REM amountwas 0.0002 mass % in No. 20 of Table 2, both of which were less than0.0003 mass % resulting in 2.3×10¹⁰ to 2.9×10¹⁰ [/mm³] of fine TiSexisted as a result of evaluation of inclusions in the steel sheets bythe above-described method showing that fixation of S by REM wasinsufficient in these cases.

The grain sizes after annealing remained between approximately 33 [μm]and 36 [μm], and the W15/50 value was approximately 3.0 [W/kg].

The above exemplary results were for the case where the stress reliefannealing was performed for a shorter period than that of the typicallyperformed stress relief annealing. When conventional stress reliefannealing is applied, a difference of grain growth due to pinning effectby fine inclusions can become more prominent.

below, certain reasons for limitation of preferable contents for thecomposition of components in exemplary embodiments of the presentinvention are described.

[C]: C has detrimental effects on a magnetic property, and magneticaging becomes remarkable due to precipitation of C, so its upper limitis set to 0.01 mass %. Its lower limit includes 0 mass %.

[Si]: Si is an element that may reduce the core loss. If its content isless than the lower limit of 0.1 mass %, core loss deteriorates. A morepreferable lower limit is 1.0 mass % in a viewpoint of further reducingcore loss. Its preferable lower limit is 0.3 mass %, more preferably 0.7mass %, and further preferably 1.0 mass %. If its content exceeds theupper limit of 7.0 mass %, the workability remarkably becomes poor, sothe upper limit is set to 7.0 mass %. Note that a preferable value asthe upper limit is 4.0 mass % where cold-rolling property is better, amore preferable value is 3.0 mass %, and a further preferable value is2.5 mass %.

[Al]: Al is an element that may reduce, similarly to Si, the core loss.If its content is less than 0.1 mass %, core loss property deteriorates,while if the content exceeds the upper limit of 3.0 mass %, the costremarkably increases. The lower limit of Al is set, in terms of coreloss, to preferably 0.2 mass %, more preferably 0.3 mass %, and furtherpreferably 0.6 mass %.

[Mn]: To increase the hardness of the steel sheet and improve itspunching property, 0.1 mass % or more of Mn is added. Note that theupper limit of 2.0 mass % is for an economical reason.

[S]: S forms a sulfide such as MnS or TiS to deteriorate the graingrowth and core loss. In the present invention, S is scavenged as REMinclusions, but its upper limit for actual use is set to 0.005 mass %,and more preferably 0.003 mass %. Its lower limit includes 0 mass %.

[N]: N forms a nitride such as AIN or TiN to deteriorate core loss. Inthe present invention, N is scavenged as TiN by REM inclusions, but itsupper limit for actual use is set to 0.005 mass %. The upper limit is,for the above-described reason, preferably 0.003 mass %, more preferably0.0025 mass %, and further preferably 0.002 mass %. Further, the amountof N is preferably provided as small as possible for the aforementionedreason. However, there may be industrial constraint to bring its contentto be as little as possible, e.g., close to 0 mass %, so the lower limitis set to greater than 0 mass %. Using the guide of the lower limit foractual use being 0.001 mass %, the content of N can be reduced to 0.0005mass %, which is preferable because formation of nitride is suppressed,and the content reduced down to 0.0001 mass % can also be preferable.

[Ti]: Ti forms fine inclusions such as TiN or TiS to deteriorate thegrain growth and core loss. In the present invention, Ti is scavenged asTiN by REM inclusions, but its upper limit for actual use is set to 0.02mass %. Note that the upper limit is, for the above-described reason,preferably 0.01 mass %, and more preferably 0.005 mass %.

As described above, the lower limit is greater than 0 mass %. When Ti isdeficient in amount, no scavenge effect is exerted on the REMinclusions. The Ti amount exceeding 0.0012 mass % is preferable becausethe scavenge effect is exerted on the REM inclusions, the Ti amountexceeding 0.0015 mass % is preferable because the scavenge effect isenforced, the Ti amount of 0.002 mass % or greater is more preferable,and the Ti amount of 0.0025 mass % or greater is further preferable.

[REM]: REM forms oxysulfide or sulfide to fix or immobilize S, therebypreventing or suppressing formation of fine sulfide. Further, REM servesas a site for composite precipitation of TiN to exert the Ti scavengeeffect. When the content is less than the lower limit of 0.0003 mass %,the above-described effect is not sufficiently exerted, while when thecontent is greater than the upper limit of 0.05 mass %, grain growth isinhibited by the contained REM inclusions. Therefore, the appropriaterange is set to not less than 0.0003 mass % nor more than 0.05 mass %.

Further, the above effect can be exerted even when only one kind or twoor more kinds of elements are used as long as they are elements includedin REM and their contents fall within the ranges of the presentinvention.

The effect of fixing S increases in proportion to the REM amount, andtherefore the lower limit of REM can be preferably 0.001 mass % orgreater, more preferably 0.002 mass % or greater, further preferably0.0025 mass % or greater, and still further preferably 0.003 mass % orgreater.

As described above, TiN is formed and grown on REM oxysulfide or REMsulfide, whereby Ti is scavenged. Accordingly, it is obvious that as theREM amount increases with respect to the Ti amount, the REM oxysulfideor REM sulfide as the TiN formation site increases more and the aboveeffect is therefore enhanced.

The ratio of the REM amount to the Ti amount, e.g., the [REM]/[Ti] valueexceeding 0.25 is enough for practical use, and if the [REM]/[Ti] valueexceeds 0.5, the above effect is preferably enhanced, the [REM]/[Ti]value exceeding 1.0 is more preferable, and the [REM]/[Ti] valueexceeding 1.25 is further preferable.

Elements other than the components described above may be contained,e.g., when they do not significantly interfere with the effect of thesteel of the present invention and fall within the scope of the presentinvention.

Selectable element will be described below. Note that all of the lowerlimits of their contents are set to greater than 0 mass % because theseelements are only required to be contained, even in a small amount.

[P]: P increases the strength of the material improving its workabilityalthough its content is preferably 0.1 mass % or less because excessiveP deteriorates cold-rolling property.

[Cu]: Cu improves the corrosion resistance and increases the resistivityimproving core loss although its content is preferably 0.5 mass % orless because excessive Cu impairs the surface quality due to occurrenceof scab and the like on the surface of a product sheet.

[Ca] and [Mg]: Ca and Mg are desulfurizing elements and combines S inthe steel to form sulfides fixing S. However, unlike REM, they are lesseffective for making composite precipitation with TiN. Thedesulfurization effect is enhanced when the adding amount is increased,but when the amount exceeds the upper limit of 0.05 mass %, excessivesulfide of Ca and Mg inhibit grain growth. Accordingly, the amount ispreferably 0.05 mass % or less.

[Cr]: Cr improves the corrosion resistance and increases the resistivityimproving core loss. However, excessive addition thereof leads to anincreased cost, and therefore the upper limit is set to 20 mass %.

[Ni]: Ni fosters the formation of texture advantageous to a goodmagnetic property improving core loss although excessive additionthereof leads to an increased cost, and therefore the upper limit is setto 1.0 mass %.

[Sn] and [Sb]: Sn and Sb are segregation elements, hinder formation oftexture on the (111) surface which deteriorates a magnetic propertyimproving core loss. These elements exert the above-described effecteither by use of only one of them or by use of two in combination. Theupper limit is set to 0.3 mass % because the content exceeding 0.3 mass% deteriorates the cold-rolling property.

[Zr]: Zr, even in a small amount, hinders grain growth to deterioratethe core loss after stress relief annealing. Accordingly, Zr ispreferably reduced as little as possible to be 0.01 mass % or less.

[V]: V forms nitride or carbide hindering magnetic domain wall motionand grain growth. Therefore, its content is preferably to 0.01 mass % orless.

[O]: When O greater than 0.005 mass % is contained, many oxides areprecipitated hindering magnetic domain wall motion and grain growth.Accordingly, its content is preferably set to 0.005 mass % or less.

[B]: B is a grain boundary segregation element and forms nitride. Thenitride hinders grain boundary migration to deteriorate core loss.Accordingly, its content is preferably reduced as much as possible to be0.005 mass % or less.

In addition to the above elements, other known elements can be addedand, for example, Bi and Ge can be used as the elements to improve amagnetic property and may be selected as required according to thedesirable magnetic property.

Further, preferable manufacturing condition in an exemplary embodimentof the present invention and the reason for defining the condition aredescribed. As an initial matter, in the steelmaking stage, it ispreferable that in a refining operation in a converter and a secondaryrefining furnace by a conventional method, the total mass percentage ofFeO and MnO in slag, referenced as an oxidation ratio, can be set to bewithin approximately 1.0% to 3.0%.

This may be because (i) when the oxidation ratio is less than 1.0%, itis difficult to effectively prevent the Ti contamination returned fromslag (formation of metal Ti due to reduction). This is due to theactivity of Ti increasing because of Si being within the Si content inelectrical steel resulting in undesired Ti increase, and (ii) when theoxidation ratio is greater than 3.0%, REM in the molten steel isundesirably oxidized due to supply of oxygen from the slag failing toform REM oxysulfide or REM sulfide, resulting in insufficient fixationof S in the exemplary steel sheet.

Further, it may also be important to eliminate or reduce externaloxidation sources as much as possible by carefully selecting arefractory for a furnace material. Furthermore, it is preferable tokeep, e.g., ten minutes or more between REM addition and casting inorder to ensure enough time to float up for the REM oxide which likelyprecipitates during the addition of REM. With the above-describedmeasures, the exemplary steel which has a composition within theintended range can be manufactured.

After producing molten steel with a composition within the desired rangeby the aforementioned method, the cast steel such as a slab or the likemay be cast by continuous casting or ingot casting.

During the casting, TiN can be precipitated on REM oxysulfide or REMsulfide making a complex. Thus, it can be important to avoid unnecessaryhigh cooling rate in casting to ensure the time enough for the growth ofTiN precipitation on them, and further to obtain the number density ofinclusions having the size defined in the present invention. Indeed, itmay be important to appropriately adjust the time for the cast steel tobe kept within the temperature range of 1200° C. to 1300° C., that is,the starting temperature of precipitation of TiN. It is significant thatalthough TiN is precipitated when the melt steel of a desiredcomposition reaches the precipitation starting temperature of TiN from ahigher temperature, the precipitation of TiN on the inclusionscontaining REM cannot grow sufficiently if the steel melt rapidly passesthrough the temperature range of 1200° C. to 1300° C., leading toinsufficient scavenging. Once the scavenging is failed, Ti precipitatesas inclusions such as TiS or TiC at a temperature lower than TiN andthey become fine inclusions after re-melting and re-precipitation by thethermal treatment in the subsequent process. Accordingly, thetemperature control when the steel passes through the aforementionedtemperature range for the first time is important.

Although the optimal temperature pattern is variously differentdepending on the composition of a steel to be manufactured, theexemplary steel should be maintained at least for, e.g., one minute ormore, preferably five minutes or more, and more preferably 20 minutes ormore within the range of 1200° C. to 1300° C., the starting temperatureof precipitation of TiN. For the method of measuring the temperature ofthe steel, a measurement such as using a radiation thermometer or acalculation analysis using heat transfer calculation can be applicable.

In the abovementioned Table 2, sample Nos. 11 and 12 are the steelswhich were allowed to pass through the temperature range of 1200° C. to1300° C. for one minute or more and less than 20 minutes. Whereas in theprocess of the sample Nos. 13 and 14, the temperature patterns wereadjusted such that the steels were gradually cooled for the periods ofseveral times more than the aforementioned period, showing that thegrain sizes and the core losses after stress relief annealing werefurther improved.

As a result of another evaluation of inclusions with a size of less than50 nm, finer than the 100 nm used in the evaluation of the table, thenumber densities of inclusions with a size of less than 50 nm includedin the product samples of Nos. 13 and 14 were 2.1×10⁹ [/mm³] and 2.3×10⁹[/mm³], respectively, both of which were not greater than 2.5×10⁹[/mm³]. In particular, with a longer period where the steel is keptwithin the temperature range of 1200° C. to 1300° C., theabove-described effect of scavenging Ti can become more remarkable andthe number density of fine inclusions with a size of less than 50 nm isreduced, leading to an improved product property.

The above-described period where the steel is maintained within thetemperature range of 1200° C. to 1300° C. is an example, and in no waylimiting.

There are various methods of adjusting the period keeping the cast steelwithin the range of 1200° C. to 1300° C. depending on the castingfacilities. The adjustment can be performed using certain equipment forkeeping the cast steel warm but can also be performed, even without suchkeep-warm equipment, for example through adjustment of flow rate ofcooling water, or adjustment of casting size or casting speed.

Subsequently, the steel can be further subjected to hot rolling, hotband annealing as required, cold rolling in one or more steps withintermediate annealing there between to a product thickness, then finishannealing, and is coated with an insulative coating. Using theabove-described method, the inclusions in the product sheet can becontrolled to fall within the scope of the present invention.

EXEMPLARY EMBODIMENT OF THE PRESENT INVENTION

Certain steel sheets contain, by mass %, C: 0.0024%, Si: 2.1%, Mn: 0.2%,and S: 0.0025%, the elements shown in Table 2, and further contain P:0.02% and Cu: 0.01%, were melted and refined to continuously cast slabs.The period during which the temperature of the slabs were lowered from1300° C. to 1200° C. was adjusted to three minutes, and the slabs werethen subjected to hot rolling, hot band annealing, and cold rolling tocold-rolled sheets with a thickness of 0.5 mm.

Subsequently, the sheets were subjected to finish annealing at 850° C.for 30 seconds and coated with an insulative coating to product sheets,and further subjected to stress relief annealing at 750° C. for 1.5hours. Then, the evaluation for inclusion, grain size, and magneticproperty evaluation of the product sheets by 25 cm Epstein's method wereperformed. The evaluation on inclusion was performed in theabove-described manner. In evaluation of the grain size, a cross sectionof the sheet perpendicular to the thickness direction was polished intoa mirror face and subjected to etching using nital to thereby allowgrains to appear so that their size was measured to obtain their averagegrain size.

As provided in the above-described Table 2, the exemplary product sheetsaccording to an exemplary embodiment of the present invention providedgood results in grain growth and core loss. On the other hand, theproduct sheets out of the range of the preset invention presentedresults inferior in grain growth and core loss.

INDUSTRIAL APPLICABILITY

According to the present invention, the size and the number density offine inclusions contained in a non-oriented electrical steel can beadjusted to fall within appropriate ranges, whereby a sufficientlyexcellent magnetic property may be obtained even by a simpler annealing.In particular, it becomes possible to obtain a sufficiently excellentmagnetic property even by a simpler stress relief annealing,contributing to saving energy consumption while satisfying the needs ofcustomers and users.

TABLE 1 Density (a) of Density (b) of Density of Core loss inclusionswith inclusions with inclusions with Recrystallized after Density ofdiameters diameters diameters grain size annealing: inclusions less than50 nm less than 100 nm 100 nm or more after annealing W15/50 No.[×10¹⁰/mm³] [×10¹⁰/mm³] [×10¹⁰/mm³] [×10¹⁰/mm³] [μm] [W/kg] Evaluation 11.55 1.01 1.39 0.16 35 2.96 X 2 1.12 0.75 1.03 0.09 47 2.87 X 3 0.970.59 0.86 0.11 64 2.69 ◯ 4 0.96 0.51 0.88 0.08 65 2.70 ◯ 5 0.75 0.280.62 0.13 66 2.67 ◯ 6 0.62 0.24 0.59 0.03 71 2.61 ⊚ 7 0.50 0.20 0.310.19 71 2.60 ⊚

TABLE 2 Major Major inclusions inclusions Recrystal- with Amount of withlized diameters inclusions with diameters grain Core loss Value of ofless diameter of less 100 nm or size after Product compositionevaluation than 100 than 100 nm in more in after annealing (mass %)equation nm in product product annealing W15/50 No. Al Ti N REM (※)product [×10¹⁰/mm³] (※※) [μm] [W/kg] Remarks 11 0.32 0.0016 0.00190.0045 0.15 MnS 0.6 REM-OS + TiN 67 2.65 within invention range 12 0.300.0012 0.0024 0.0296 0.04 Cu₂S 0.5 REM-OS + TiN 68 2.64 within inventionrange 13 0.30 0.0020 0.0011 0.0013 0.33 Cu₂S 0.3 REM-OS + TiN 71 2.61within invention range 14 0.31 0.0015 0.0008 0.0069 0.21 Cu₂S 0.4REM-OS + TiN 69 2.62 within invention range 15 0.31 0.0228 0.0020 0.00721.32 TiC 2.5 REM-OS + TiN 35 3.06 excessive Ti 16 0.31 0.0012 0.00430.0051 −0.03 TiN 1.8 REM-OS, 38 2.83 out of AlN evaluation equation 170.30 0.0011 0.0029 0.0051 −0.01 TiN 1.7 REM-OS, 39 2.81 out of AlNevaluation equation 18 0.30 0.0009 0.0022 0.0034 −0.08 TiN 1.6 REM-OS,41 2.76 out of AlN evaluation equation 19 0.32 0.0016 0.0024 — 0.13 TiS2.9 MnS, AlN 33 3.11 no REM 20 0.32 0.0024 0.0021 0.0002 0.32 TiS 2.3REM-OS + 36 2.99 deficient TiN, MnS REM (※) log([Ti] × [N]) − 1.19 ×log([Al] × [N]) + 1.84 (※※) REM-OS + TiN: REM oxysulfide + TiN composite

1. A non-oriented electrical steel sheet, comprising: a particularnumber density of inclusions with an equivalent volume diameter of lessthan 100 nm contained in the steel sheet, the particular number densityof inclusions being approximately at most 1×10¹⁰ [/mm³]; and comprising,by mass %: C: up to 0.01%, Si: 0.1% to 7.0%, Al: 0.1% to 3.0%, Mn: 0.1%to 2.0%, REM: 0.0003% to 0.05%, Ti: up to 0.02%, S: up to 0.005%, N: upto 0.005%, and a balance of Fe and inevitable impurities, and whereinthe mass % of Al, the mass % of N and the mass % of Ti satisfy thefollowing equation:log([Ti]×[N])−1.19×log([Al]×[N])+1.84>0.
 2. A non-oriented electricalsteel sheet, comprising: a particular number density of inclusions withan equivalent volume diameter of less than 50 nm contained in the steelsheet, the particular number density of inclusions being approximatelyat most 2.5×10⁹ [/mm³]; and comprising, by mass %: C: up to 0.01%, Si:0.1% to 7.0%, Al: 0.1% to 3.0%, Mn: 0.1% to 2.0%, REM: 0.0003% to 0.05%,Ti: up to 0.02%, S: up to 0.005%, N: up to 0.005%, and a balance of Feand inevitable impurities, and wherein the mass % of Al, the mass % of Nand the mass % of Ti satisfy the following equation:log([Ti]×[N])−1.19×log([Al]×[N])+1.84>0.
 3. The non-oriented electricalsteel sheet according to claim 1, further comprising, by mass %: atleast one P: up to 0.1%, Cu: up to 0.5%, Ca or Mg: up to 0.05%, Cr: upto 20%, Ni: up to 1.0%, at least one of Sn or Sb: up to 0.3%, Zr: up to0.01%, V: up to 0.01%, O: up to 0.005%, and B: up to 0.005%.
 4. Thenon-oriented electrical steel sheet according to claim 2, furthercomprising, by mass %: at least one P: up to 0.1%, Cu: up to 0.5%, Ca orMg: up to 0.05%, Cr: up to 20%, Ni: up to 1.0%, at least one of Sn orSb: up to 0.3%, Zr: up to 0.01%, V: up to 0.01%, O: up to 0.005%, and B:up to 0.005%.
 5. A method for manufacturing a non-oriented electricalsteel sheet, comprising: maintaining the steel sheet within atemperature range of 1200° C. to 1300° C. for approximately at least oneminute, the steel sheet containing, by mass %: C: up to 0.01%, Si: 0.1%to 7.0%, Al: 0.1% to 3.0%, Mn: 0.1% to 2.0%, REM: 0.0003% to 0.05%, Ti:up to 0.02%, S: up to 0.005%, N: up to 0.005%, and a balance of Fe andinevitable impurities, and wherein the mass % of Al, the mass % of N andthe mass % of Ti satisfy the following equation:log([Ti]×[N])−1.19×log([Al]×[N])+1.84>0.
 6. The method according toclaim 5, wherein the steel sheet further comprises, by mass %, at leastone P: up to 0.1%, Cu: up to 0.5%, at least one of Ca or Mg: up to0.05%, Cr: up to 20%, Ni: up to 1.0%, at least one of Sn and Sb: up to0.3%, Zr: up to 0.01%, V: up to 0.01%, O: up to 0.005%, and B: up to0.005%.